Recent development in the field of polyimides may be traced to the need felt in aeronautics and space research for polymers with enhanced high temperature capabilities. Linear systems developed in the early research were impossible to process in high molecular weight imide form and were processed as polyamic acids, soluble only in high boiling solvents such as dimethylformamide (DMF), N-methylpyrrolidone (NMP), etc. which could not be easily removed because of their strong association with the polymer. Properties of the resultant structure also often suffered due to the presence of voids. Over the past decade, aromatic addition polyimides have been developed both as matrix and adhesive resins for applications on future aircraft and spacecraft. Additions polyimides offer distinct advantages over linear polyimides being processable in the form of short chained oligomers end-capped with latent crosslinking groups. By polymerizing this type of material "in place" it is useful as an adhesive. This same processing method may be used for the preparation of fiber reinforced composites where the polymerization encapsulates the reinforcement and is set.
One particularly attractive addition-type polyimide that is commercially available is Gulf Oil Chemical's Thermid 600 (now produced by National Starch) which has the following structure: ##STR1## This polyimide is prepared by the reaction of an aromatic tetracarboxylic acid dianhydride with an aromatic diamine in the presence of an ethynyl-substituted aromatic monoamine to provide a fully imidized acetylene-terminated thermosetting polyimide resin. Unfortunately it is insoluble in all solvents except NMP at 100.degree. C. leading back to the problems associated with this high boiling point polar solvent.
Addition polyimides of this type also tend to be highly crosslinked, insoluble and extremely brittle on curing. Efforts made to toughen addition polyimide adhesives for example, LARC-13, by physically blending fluorosilicone and vinyl terminated silicone rubbers have been partially successful as disclosed in U.S. Pat. No. 4,389,504.
There is thus a definite need in the art to toughen addition polyimides to provide such properties as increased peel strength, resistance to adhesive fracture and higher impact energy. There is also need for addition polyimides with improved solubility and processability.
Accordingly, an object of the present invention is the synthesis of addition-type polyimides with improved toughness.
Another object is the synthesis of an addition-type polyimide with improved solubility.
Another object of the present invention is a process for reducing the polymerization temperature of addition-type polyimides.
Another object of the present invention is a novel polyimide having a reduced melting temperature.
Another object of the present invention is a novel process for preparing scrim cloths by "melt-impregnation" without the use of additional solvents.
Another object of the present invention is to utilize the polymer as an adhesive in bonding various surfaces.
Another object of the present invention is a novel process of preparing prepregs for use in fabricating composite structures.